Antagonistic interactions of Pseudomonas aeruginosa antibiotic resistance mechanisms in planktonic but not biofilm growth.

Pseudomonas aeruginosa has an extraordinary capacity to evade the activity of antibiotics through a complex interplay of intrinsic and mutation-driven resistance pathways, which are, unfortunately, often additive or synergistic, leading to multidrug (or even pandrug) resistance. However, we show that one of these mechanisms, overexpression of the MexCD-OprJ efflux pump (driven by inactivation of its negative regulator NfxB), causes major changes in the cell envelope physiology, impairing the backbone of P. aeruginosa intrinsic resistance, including the major constitutive (MexAB-OprM) and inducible (MexXY-OprM) efflux pumps and the inducible AmpC β-lactamase. Moreover, it also impaired the most relevant mutation-driven β-lactam resistance mechanism (constitutive AmpC overexpression), through a dramatic decrease in periplasmic β-lactamase activity, apparently produced by an abnormal permeation of AmpC out of the cell. While these results could delineate future strategies for combating antibiotic resistance in cases of acute nosocomial infections, a major drawback for the potential exploitation of the described antagonistic interaction between resistance mechanisms came from the differential bacterial physiology characteristics of biofilm growth, a hallmark of chronic infections. Although the failure to concentrate AmpC activity in the periplasm dramatically limits the protection of the targets (penicillin-binding proteins [PBPs]) of β-lactams at the individual cell level, the expected outcome for cells growing as biofilm communities, which are surrounded by a thick extracellular matrix, was less obvious. Indeed, our results showed that AmpC produced by nfxB mutants is protective in biofilm growth, suggesting that the permeation of AmpC into the matrix protects biofilm communities against β-lactams.